使用 for 循环遍历字典产生的疑问

Question

下面的代码让我有点困惑：

d = {'x': 1, 'y': 2, 'z': 3} 
for key in d:
    print key, 'corresponds to', d[key]

我对key部分不是很理解。Python 如何识别它只需要从字典中读取密钥？在 Python 中，key 是一个特殊的单词吗? 还是仅仅是一个变量？

Answer 1

key is just a variable name.

for key in d:

will simply loop over the keys in the dictionary, rather than the keys and values. To loop over both key and value you can use the following:

For Python 2.x:

for key, value in d.iteritems():

For Python 3.x:

for key, value in d.items():

To test for yourself, change the word key to poop.

For Python 3.x, iteritems() has been replaced with simply items(), which returns a set-like view backed by the dict, like iteritems() but even better. This is also available in 2.7 as viewitems().

The operation items() will work for both 2 and 3, but in 2 it will return a list of the dictionary's (key, value) pairs, which will not reflect changes to the dict that happen after the items() call. If you want the 2.x behavior in 3.x, you can call list(d.items()).